What happens when a little false teaching is allowed into a church?
A little false teaching acts like leaven: it does not stay contained. Once admitted, it spreads through the whole church, one error leading to another, until the entire body is permeated. Over time, the church accommodates worldly things and becomes indistinguishable from the world. This is what happens when a little leaven leavens the whole lump.
Les teaches that leaven is yeast, and yeast put into dough does not leaven only a fourth of it but leavens all of it. The same is true with false teaching: if a little bit of false teaching is let in as leaven, it is not long until it permeates the whole, and one error leads to another. He points to Christendom in general as having been leavened, so that Christianity today is leavened; things Scripture and Christianity stood against fifty, sixty, or seventy years ago are now part and parcel of everyday experience. He illustrates this with a church they visited where the world came knocking, and the church said it could accommodate that; soon the door opened further, more worldly things came in, and the church said it could live with that too. Now the doors are wide open, and you cannot tell the difference between the church and the world. A little leaven leavened the whole lump. Yet Paul did not give up on the people, but had confidence in them and was trusting God to bring them back to the truth.
A little leaven leaveneth the whole lump.